The present invention relates to closed-center fluid controllers, and more particularly, to such controllers which are to be used with unequal area steering cylinders.
In many vehicle steering systems, closed-center controllers are now used rather than open-center controllers, primarily because a closed-center controller, fed by a variable displacement pump, does not consume as much engine horsepower as does an open-center controller, fed by a fixed displacement pump.
Typically, vehicle steering systems including closed-center controllers have used double rod end, equal area steering cyllnders. In most such systems, it would be more desirable to utilize an unequal area cylinder, primarily because such cylinders are cheaper, and lend themselves to simpler and less expensive installation on the vehicle.
However, there are a number of vehicles, such as loader-backhoes, where it has not been considered possible to combine an unequal area cylinder and a closed center controller for various reasons. As one example, in a loader-backhoe, the steering wheels may be raised off the ground while certain vehicle implements are being used. It has been found that the buildup of residual pressure, due to leakage, in the steering system will cause extension of the piston rod of an unequal area cylinder, thus changing the orientaion of the steered wheels. If this occurs without the operator being aware of the change, an objectionable condition may exist when the steered wheels again engage the ground and the vehicle begins to move.
Accordingly, it is an object of the present invention to provide an improved closed-center controller which may be used with an unequal area steering cylinder without the buildup of residual pressure in the system and the resulting undesired piston rod extension.
The above and other objects of the present invention are accomplished by the provision of an improved closed-center controller of the type including a housing having a fluid inlet port, a fluid return port, and a pair of control fluid ports adapted for connection to an unequal area piston-cylinder fluid pressure device. A valve means disposed within the controller housing includes a primary rotatable valve member and a relatively rotatable follow-up valve member which define a neutral position and at least one operating position. One of the valve members and the housing cooperate to define an inlet fluid region in communication with the inlet ports, an at least one control fluid region in communication with one of the control fluid ports.
The controller is characterized by the follow-up valve member and the housing cooperating to define a generally annular leakage fluid chamber disposed axially between the inlet fluid region and the control fluid region to interrupt the flow of leakage fluid from the inlet fluid region to the control fluid region between the housing and the follow-up valve member. Neutral drain passage means is disposed to provide communication between the leakage fluid chamber and the fluid return port when the valve means is in the neutral position, and flow through the neutral drain passage means is blocked when the valve means is in the operating position .